ASTM D6443 Copper, Phosphorus and Sulfur in Unused Lubricating Oils and Additives
ASTM D6443 Standard Test Method for Determination of Calcium, Chlorine, Copper, Magnesium, Phosphorus, Sulfur, and Zinc in Unused Lubricating Oils and Additives by Wavelength Dispersive X-ray Fluorescence Spectrometry (Mathematical Correction Procedure)
3. Summary of Test Method
3.1 The X-ray fluorescence spectrometer is initially calibrated by the following procedure. For each element, the slope and intercept of the calibration curve are determined by regressing concentration data and intensities measured on a set of physical standards. Empirical alphas can also be determined by regression when the appropriate set of physical standards is used for calibration. Theoretical alphas, calculated with special software, can also be used. In addition, a combination of theoretical and empirical alphas can be used.
3.2 A sample is placed in the X-ray beam, and the intensities of the appropriate fluorescence lines are measured. A similar measurement is made at a wavelength offset from each fluorescence line in order to obtain a background correction. Enhancement or absorption of the X-ray fluorescence of an analyte by an interfering element in the sample can occur, and these effects can be handled in the data reduction by implementation of alphas. Concentrations of the analytes are determined by comparison of net signals against calibration curves, which include influence coefficients (that is, alphas) calculated from theory, empirical data, or a combination of theory and empirical data.
4. Significance and Use
4.1 Lubricating oils can be formulated with additives, which can act as detergents, anti-oxidants, anti-wear agents, and so forth. Some additives can contain one or more of calcium, copper, magnesium, phosphorus, sulfur, and zinc. This test method can be used to determine if the oils, additives, and additive packages meet specification with respect to content of these elements.
4.2 This test method can also be used to determine if lubricating oils, additives, and additive packages meet specification with respect to chlorine concentration. In this context, specification can refer to contamination.
4.3 This test method is not intended for use on samples that contain some component that significantly interferes with the analysis of the elements specified in the scope.
4.4 This test method can complement other test methods for lube oils and additives, including Test Methods D4628, D4927, D4951, and D5185.
5. Interferences
5.1 The additive elements can affect the magnitudes of the measured intensities for each analyte. In general, the X-radiation emitted by each analyte can be absorbed by the other elements. Also, the X-radiation emitted by an analyte can be enhanced by some other component. The magnitudes of the absorption and enhancement effects can be significant. However, implementation of accurately determined alphas in the set of calibration parameters can satisfactorily correct for absorption and enhancement effects, thereby making this test method quantitative.
5.2 Molybdenum lines can spectrally overlap lines of magnesium, phosphorus, sulfur, and chlorine. Lead lines can spectrally overlap sulfur. Thus, this test method cannot be applied if molybdenum or lead are present at significant concentrations and if accurate overlap corrections cannot be made.
5.3 When a large d-spacing diffraction structure containing silicon is used as the analyzing crystal, corrections for the fluorescence of silicon may be needed. Calcium X rays from sample specimens cause silicon to fluoresce. This silicon radiation contributes to fluctuations in the background for magnesium measurements. If the effect is significant, this interference may be treated as a line overlap due to calcium.
